The concept of valence and conduction band broadening based on alternating electron rich and poor moieties in conjugated polymers was introduced in 1993 by Havinga et al. With the ability to obtain energy gaps as narrow as 0.5 eV, a new dimension was added to the notion of band gap engineering and the “donor-acceptor” theory on a macromolecular level was born. In spite of the considerable impact of this discovery for device applications such as field-effect transistors, light emitting diodes and photovoltaics, only recently has the use of donor-acceptor based conducting polymers been directed to electrochromic polymers (ECPs).
ECPs have a rather unique combination of mechanical flexibility, high contrast ratios, fast response times, and the inherent potential for fine color tunability by control of the structure. Polymeric electrochromes are the most attractive alternatives to metal-oxides (MOs) for both reflective and absorptive/transmissive electrochromic devices (ECDs) and displays.
Recently, the focus of developments has been directed generally to incorporating ECPs as fast and reversible color changing materials in devices such as windows, mirrors and displays. While a number of red, blue and recently green (RGB) candidates for completion of the additive primary color space were reported, attempts in making saturated black polymers have not been successful due to the complexity in designing materials that absorb effectively over the entire visible spectrum. Such saturated black ECPs would be highly valuable for the fabrication of polymer-based solar cells (PSCs) where efforts continue to identify and prepare materials that absorb homogeneously over a broad bandwidth of the UV-visible spectrum.
Strongly absorbing ECPs, which can switch to a highly transmissive state, have remained a challenge, primarily because of the difficulty of achieving simultaneous and efficient bleaching of all absorption bands over the visible region. Generally, as the doping level induced by chemical or electrochemical redox control is progressively increased in ECPs, charged carriers balanced with counter ions arise along the backbone, changing the intrinsic optical transitions of the polymer. Sufficiently low energy-gap polymers undergo oxidative (p-type) doping with the formation of radical cations (polarons) and dications (bipolarons) that absorb in the near-IR, depleting the ground-state optical transitions of the electroactive material in the visible region.